1. Technical Field
The present invention relates to structural connections between thermosetting polymer and thermoplastic polymer or aluminium elements, and methods of forming such structural connections.
The method of the invention is particularly, but not exclusively, suitable for connecting end caps to polymer tubes to form component members for the construction of structures.
2. Prior Art
Spaceframes are usually built up from simple, prefabricated units (i.e. members and nodes), which are often of standard size and shape. Such units are mass produced in the factory and can easily and rapidly be assembled on site by semi-skilled labour. The small size of the components greatly simplifies handling, transportation and erection.
Spaceframe roofs are extremely rigid and stiff, and have an exceptional ability to resist large, concentrated or asymmetrical loading. It has been shown that, even when badly damaged, double layer grids never collapse rapidly; this is of particular importance in the case of fire.
Load-bearing spaceframes have conventionally been constructed of steel or aluminium component members and nodes. However, in order to provide a less expensive spaceframe structure particularly suitable for smaller roofing spans of, for example, less than 10 to 15 meters, it would be desirable to use polymer materials.
If the manufacturing costs of a lightweight plastics spaceframe are to be low, the structure must make use of injection moulded nodes. Polymers suitable for such injection mouldings would normally be of the thermoplastics variety while those suitable for the tubular members of the spaceframe would be of the thermosetting variety (for example, polyester). There are considerable differences between these two types; for example, thermoplastics are usually weaker and are more difficult to chemically bond than thermosetting polymers.
A suitable material for the tubular members would be pultruded G.R.P. (glass reinforced polyester). Tubes of this material have excellent strength and stiffness properties, but it has always been exceptionally difficult to transfer loads from the material. If the tubes are drilled to receive bolted connections, the continuous glass fibres are broken and the material severely weakened; the bolts simply shear through the tubes.
It is not possible to weld the material, and the process of threading the tubes again merely breaks the load-carrying glass fibres. The only practical method of utilizing the full strength of the material is to transfer load by adhesive bonding. The main problem with the use of adhesives, however, is the control of the glue line thickness; this is critical if a good bond is to be achieved. If the glue line is too thick then the adhesive will shear through at a low load; if the glue line is too thin, then there is the risk that, in places, no adhesive exists at all. In addition it is generally considered by the construction industry that adhesives are not suitable outside the controlled conditions of factories.